Electrochemical synthesis of mesoporous metal/metal oxide flims using a low percentage surfactant solution by cooperative templating mechanism

ABSTRACT

An electrochemical method for generating both mesoporous metal and metal oxide films from dilute surfactant solutions by utilizing self assembly of surfactant-inorganic aggregates in the electric field of the solid-liquid interface at an electrode to specifically direct the morphology of the film. The surface of the working electrode serves as a solid-liquid interface in a plating solution containing surfactant and inorganic ions and salts.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0001] This invention was made with Government support under Grant No. DMR 9634396, awarded by the National Science Foundation. The Government has certain rights in this invention.

FIELD OF THE INVENTION

[0002] The invention relates to the preparation of mesoporous metal/metal oxide films.

BACKGROUND OF THE INVENTION

[0003] Over recent years materials with pore sizes in the mesoporous domain (2-50 nm according to the IUPAC convention) have attracted considerable interest because of their potential application in catalysis and chemical separations due to their high surface areas and large pore sizes. During the last decade, extensive work has been performed on the synthesis of mesoporous silica using self assembly of surfactants,¹ block copolymers,^(2,3) colloidal suspensions,⁴ and proteins.⁵ However, syntheses of the surfactant templating for the formation of non-silica mesoporous oxides has not been successful, although these materials possess more potential for applications (i.e. high surface area battery electrode, sensors, electro-optical and electrochemical devices, and catalysts). This is mainly due to the instability of these materials upon calcination and/or removal of structure directing agents. Only a few materials have been prepared that exhibit satisfactory thermal stability after calcination.^(6,7)

[0004] For the sensing and electrochemical applications, these materials need to be processed as thin films with the pore channels normal to the film surfaces in order to make the best use of the high surface areas and the porous structures. For example, films with cubic phases with three dimensionally interconnected pores or 2D hexagonal phases with pores perpendicular to the film surface are highly desirable, while those with 2D hexagonal phases with pores channels parallel to the plane of the films are practically useless. Unfortunately, the conventional sol-gel dip coating methods cannot generate films with 2D hexagonal structures with pores perpendicular to the film surface and it is relatively difficult to generate films with cubic phases because of a narrow range of synthetic conditions to stabilize cubic phases. Therefore, an innovative synthetic method to produce non-silica mesoporous films with thermal stability and accessible pores is highly desirable.

[0005] Recently, Attard et al. has demonstrated a possibility of electrochemically producing hexagonally ordered mesoporous metal films with the pores not necessarily parallel to the substrate.^(3,9) They cathodically deposited Pt films from Pt⁴⁺ ions in the liquid crystalline plating solution containing 42 wt % of nonionic surfactant (octaethylene glycol monohexadecyl ether, C₁₆EO₈), 29 wt % of H₂PtCl₆, and 29 wt % of water. However, no further research has been made to show that the electrochemical synthesis can be used for the production of non-silica mesoporous metal oxide films. In addition, no study has been performed to probe the possibility of generating mesoporous films from the solution containing low concentration of surfactants without pre-existing liquid crystalline phase, which will not only provide a different nanostructure building mechanism from the previous methods but also significantly reduce the cost of producing these materials.

SUMMARY OF THE INVENTION

[0006] The present invention presents a new electrochemical method to generate both mesoporous metal and metal oxide films from dilute surfactant solutions (0.1-20 wt %) by utilizing self assembly of surfactant-inorganic aggregates at solid-liquid interfaces. In this approach the surface of the working electrode serves as a solid-liquid interface in a plating solution containing surfactant. The surface charge density can be selectively controlled by a bias voltage to induce a desired surfactant-inorganic assembly on the substrate. When the assembly potential lies within the range of potentials required to reduce the necessary inorganic species, nanostructured films will be deposited as patterned by the surfactant-inorganic aggregates. Due to kinetically controlled surfactant-inorganic assembly during the deposition process, the resulting films possess considerable regions with pores or layers that can be easily accessed by guest molecules or analytes. Many of these orientations cannot be assembled by sol-gel dip coating methods. In the method of this invention, the formation of mesostructure framework is completed at the time of deposition and as-deposited films do not contain surfactant molecules. Therefore, post-synthesis aging or removal of surfactants is not necessary. This new method provides an easy, fast, inexpensive, and versatile route to the production of various metal/metal oxide films of technological importance in forms, which cannot be accessed by other means.

[0007] Due to its high surface areas mesoporous materials have excellent potential for applications as high surface area battery electrodes, electrochemical capacitors, fuel cells, sensors, luminescent devices, opto-electronic devices, and photo and chemical catalysts. By filling the pores with proper types of dyes, metals, semiconductors, or polymers, the application of the mesoporous materials can be extended to the lasers,¹⁰ sensors,¹¹ advanced photograph films and imaging plates, photovoltaics,¹² and magneto-optic/magneto-electronic devices. In addition, coupling the surfaces of these materials to biological systems (i.e. tissues and cells) can offer unique opportunities for drug delivery. Mesoporous materials are also useful as a support for subsequent deposition or incorporation of the other materials to generate quantum dots or nanowires.¹³

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is X-ray diffraction pattern of the lamellar structured ZnO film deposited with 0.1 wt % sodium dodecyl sulfate (SDS);

[0009]FIG. 2 are (a) TEM image and (b) 2D giSAXS pattern of the ZnO film deposited with 0.1 wt % sodium dodecyl sulfate;

[0010]FIG. 3 are TEM images of mesoporous Pt films deposited with 0.1 wt % SDS. (a) view of hexagonally ordered pores, (b) side view of pore channels, and (c) Pt film after heated to 500° C. for 2 hours.

[0011]FIG. 4 is cyclic voltammogram for the mesoporous Pt film in 0.5M H₂SO₄ aqueous solution at 200 mVs⁻¹, which is measured to calculate the surface area of the Pt films. H_(c): formation of adsorbed hydrogen, H_(a): oxidation of adsorbed hydrogen, O_(a): formation of adsorbed oxygen, O_(c): reduction of oxide layer, shaded area: double layer capacitive charge.

DETAILED DESCRIPTION OF THE INVENTION

[0012] Surfactants in solution spontaneously aggregate at solid-liquid interfaces due to surface forces (i.e. electrostatic interaction between the surfactant head group and a surface charge).¹⁴ Micelles can be formed at the surface even when the surfactant concentration (surface micelle concentration, smc) is lower than the critical micelle concentration (cmc) because surface forces generate surface excess of surfactant molecules.¹⁵ The assembly patterns of the surface aggregates are frequently different from those formed in free solution.¹⁶ Among the factors that affect the organization of surfactants on the surface (i.e. hydrophilicity of the substrate, surfactant types, types of counter ions, and ionic strength), surface charge density is unique in that it can be controlled externally using a bias voltage applied to the substrate. This makes it possible to selectively induce and stabilize phases of surfactant aggregates by deliberate control of the electrochemical potential.¹⁷

[0013] The present invention combines potential-controlled surface assembly and an electrodeposition process to fabricate nanostructured films. In this approach, the surface of the working electrode serves as a solid-liquid interface in a plating solution containing surfactants. When there exists a common potential that can simultaneously induce a desired phase of surfactant-inorganic aggregates and reduce the metal ions, a nanostructured metallic film will be deposited patterned by the surfactant-inorganic aggregates. Scheme 1 shows several surface assemblies of surfactants. Phases composed of hemisphere/sphere micelles and hemicylinder/cylinder micelles are suitable to fabricate mesoporous films (i.e. hexagonal, cubic, and disordered porous structures), while bilayers may result in layered structures or featureless films. Through this invention, it becomes possible to deposit high quality nanostructured films from dilute surfactant solutions without forming liquid crystalline phases of surfactant.

[0014] The method of this invention presents distinctive difference in the formation mechanism of nanostructures from those of sol-gel dip coating methods. In sol-gel dip coating, the orientation of the mesostructures (i.e. 2D hexagonal and lamellar phases) is solely determined by the thermodynamically stable packing of surfactant-inorganic aggregates when the substrate is exposed to air and the solvents is evaporated. This results in one homogeneous arrangement of one-dimensional pores or layers (i.e. 2D hexagonal phase with pores parallel to the substrate and lamellar phase with layers parallel to the substrate), which do not allow facile access of guest molecules to the pores or interlayers. In an electrodeposition process, nanostructured films are formed while the substrate remains in solution. When the electric field is applied, surfactant-inorganic assembly and deposition of the inorganic species occur simultaneously. In this situation, the arrangement of surfactant-inorganic aggregates are kinetically determined by the dynamics of charged species near the electrode and by the mechanism of the deposition processes (i.e. growth pattern of particles, deposition rate, redox mechanism). This can result in new types or orientations of nanostructures not accessible by other means (i.e. 2D hexagonal phase with pores perpendicular to the substrate or lamellar phase with the layers perpendicular to the substrate), which are more favorable for practical point of view.

[0015] Another main difference of the present invention from the sol-gel dip coating methods is the content of the surfactant in as-deposited films. In electrodeposition, as soon as the mesostructured wall is deposited, the electrostatic interactions between the inorganic ions and the surfactant head groups disappear because the deposits are neutralized by the redox reaction at the electrode. When the deposition is completed, the surfactant molecules are only physically attached to the surface and, therefore, can be easily washed away without disrupting the mesostructures. The carbon content of as-deposited films produced by this invention is less than 5%.

[0016] Typical examples of depositing (I) mesoporous metal oxide film and (II) metal film are given below.

EXAMPLE 1 Deposition of ZnO¹⁸

[0017] ZnO films were prepared by cathodic deposition from an aqueous solution composed of the ternary system Zn(NO₃)₂.6H₂O, sodium dodecyl sulfate and water. A 0.02 M aqueous solution of Zn(NO₃)₂.6H₂O was mixed with sodium dodecyl sulfate (0.1%-20 wt %) and stirred to make homogeneous solution. A classic three electrode setup was used in an undivided cell. For both working and counter electrodes, 100 Å of titanium followed by 500 Å of platinum were deposited by e-beam evaporation on the cleaned microscopic glasses. The reference electrode was Ag/AgCl electrode in saturated KCl. Electrodeposition was carried out potentiostatically without stirring at the cathodic potential of −0.3≦V≦−0.7 against the reference electrode. The plating solution was kept at 65° C during the deposition. The resulting films were washed with deionized water and dried in the air.

[0018] The small angle x-ray patters of the nanostructured ZnO film deposited with 0.1 wt % SDS is shown in FIG. 1. The diffraction patterns are unambiguously indexed as two different lamellar phases, one with d₀₀₁=31.7 Å and the other with d_(001*)=27.5 Å, implying two slightly different pathways to form stable surfactant bilayers under our deposition condition. The same d-spacings were obtained in multiple repeat experiments. Presence of high order 00/peaks for both phases are indicative of well-defined nanostructures with long range order.

[0019] Transmission electron microscopy (TEM) images of the same ZnO film confirmed a lamellar structure (FIG. 2(a)) with the interlayer spacing and the inorganic wall thickness of ˜15 Å and ˜15 Å, respectively, which corroborates well with small angle XRD results. The large domain size that shows the stacks of layers in plan-view TEM images implies considerable regions where layers are stacked not parallel to the substrate. This arrangement, which will allow facile access of the guest molecules and analytes to the interlayers, is confirmed by two-dimensional grazing-incidence small angle x-ray scatterring (giSAXS) of the same ZnO film; the borad arcs in FIG. 2(b) are indicative of a wide distribution of the stacking directions. For comparison, sol-gel dip coating methods always produce lamellar structured films with layers strictly parallel to the substrate. Carbon-Hydrogen-Nitrogen analysis for the as-deposited films shows 2.4% carbon content, indicating that the majority of surfactants can be removed by thoroughly washing the films with water and ethanol.

EXAMPLE 2 Deposition of Pt

[0020] Pt films were deposited by the cathodic reduction of Pt⁴⁺ species in an aqueous solution. The electrolyte consisted of 0.05-0.1M H₂PtCl₆ containing 0.1-5 wt % sodium dodecyl sulfate (SDS), A deposition potential of −0.2V was found to be optimal to fabricate well-ordered hexagonal structures in the platinum films. Deposition at higher or lower potentials did not generate mesoporous films due, for example, to the formation of bilayers or desorption of surfactants from the electrode. The resulting films were washed with deionized water and dried in the air.

[0021] Transmission electron microscopy (TEM) images of the platinum films revealed hexagonally ordered pores, see FIGS. 3(a) and (b). From the TEM images, both the pore size and the wall thickness were estimated to be ˜40 Å. The shapes and the sizes of domains with pores along the [100] axis in the TEM images (FIG. 3(a)) imply regions with cylindrical micelles perpendicular to the substrate during deposition. We suspect that the electric field and the corresponding dynamics of the charged particles are responsible for forming these domains¹⁹ since such alignment has not been observed by previous AFM/STM studies of SDS aggregates under static equilibrium conditions. Thermal stability of the mesostructures was examined by heating the Pt films at 500° C. for two hours. As shown in FIG. 3(c), the mesoporous structure remains intact with no visible change in the pore diameter or wall thickness.

[0022] The surface area of the mesoporous Pt film was determined using cyclovoltammetry in aqueous 0.5M H₂SO₄ solution at 200 mVs⁻¹ (FIG. 4). The fine structure between −0.2V and 0.2V corresponds to the formation and removal of the surface adsorbed hydrogen layer.²⁰ The charge associated with forming and removing such a surface layer is directly proportional to the active Pt surface area (see experimental). The specific surface area was estimated to be 47.1 m²/g, which is equivalent to 1008 cm³/m². For comparison, the specific surface areas of platinum black range from 20 to 28 m²/g.

[0023] Modifications and variations

[0024] 1. In addition to the production of ZnO and Pt metal, the methodology of the present invention is applicable to the production of a variety of mesoporous metal/metal oxide films including transition metals (e.g. Ru, Au, Ag, NiO, WO₃, ZrO₂, TiO₂, CdO, V₂O₅, Nb₂O₅), lanthanide metals (e.g. CeO₂), and main group metals (e.g. Sn, SnO₂, Pb, PbO₂). The mesoporous oxide films that can be generated by this method is not limited to the binary phases but can be extended to ternary system such as BaTiO₃ ²¹ LaMnO_(3.) ²² Based on preliminary results, any materials that can be electrochemically deposited can be fabricated as mesoporous films by the method of this invention.

[0025] 2. The surfactant templating method can also be combined with anodic deposition to produce mesoporous metal oxides that cannot be produced cathodically or that are more easily deposited anodically. Examples include Fe₂O₃, CoO₂, MnO₂, and AgO.

[0026] 3. Incorporation of dopants and/or preparing solid solution is easy to achieve by electrochemistry.^(23,24) Dopants can be simultaneously deposited as the mesoporous structures are fabricated. Therefore, it is possible to tailor the band gap or electrical conductivity of the resulting mesoporous materials as desired.

[0027] 4. As in conventional sol-gel templating synthesis, the pore size or the structure type of the mesophases can be controlled by changing types or concentrations of surfactant/block copolymers. Deposition conditions such as deposition potential, current, and deposition temperature are also considered important factors to control these structural features.

[0028] 5. The same electrochemical templating synthesis can be extended to the electrodeposition of mesoporous chalcogenide compounds (i.e. CdQ, ZnQ; Q=S, Se, Te). These materials usually exhibit much narrower band gaps than oxides and can be better candidates for many applications that require higher electrical conductivity.

[0029] 6. This method is not limited to aqueous plating solutions. Non-aqueous solvents such as ethanol, dimethyl sulfoxide, prophylene carbonate, acetonitril, and formamide can be used instead in order to produce films with different mesostructures and electrical/optical properties from those of films formed in aqueous solutions.

[0030] The method presented here has numerous advantages by comparison to currently widely used synthetic methods for mesoporous thin films, such as sol-gel dip coating method:

[0031] 1. A safe, easy, and inexpensive synthesis. This technique does not require high power source and/or high vacuum system. The materials are deposited from the aqueous solutions, which contain very low concentrations of inorganic species (≦1M) and surfactant (≦20 wt %). The voltage required to generate these materials are usually below 1V and the deposition temperature is lower than 90° C.

[0032] 2. Versatility. It is possible to apply this synthetic method to produce almost any kinds of metal/metal oxide films by tuning the cathodic potential and pH of the electrolyte bath. For the materials that does not have a window of potential for a selective deposition, corresponding metal or metal oxides can be deposited first and can be converted to the desired materials by heating them in a proper atmosphere. For example, to prepare mesoporous RuO₂, mesoporous Ru metal can be deposited first and oxidized to RuO₂ by heating it in the air. This is possible only because the method of this invention produces thermally stable mesostructures. When the post-deposition heating process is taken into consideration, the types of metal/and metal oxides that can be produced as mesoporous films by the method of this invention becomes even broader.

[0033] 3. Fast synthesis of robust mesoporous framework. The formation of mesostructure framework is completed at the time of deposition and post-deposition aging process, which is a critical step to cross-link mesostructure framework in sol-gel process, is not necessary. The as-deposited films show excellent adhesion to the substrate and can be rinsed with deionized water. Mesostructures of the films remain intact during rinsing process.

[0034] 4. Thermal stability. The mesoporous structure produced by electrochemical synthesis are thermally stable and do not collapse upon heating, which is a very important feature for many applications.

[0035] 5. No incorporation of the surfactant molecules. The surfactant molecules, which are used as structure directing agents, do not incorporate into the pores of the mesoporous films, which will eliminate calcination or other surfactant removal processes from the synthetic procedure, thus significantly reducing the amount of time and effort required to synthesize these materials.

[0036] 6. Accessible pores. Due to kinetically controlled surfactant-inorganic assembly during the deposition process, the method of this invention generates domains with many different stacking directions of pores and layers. As a result, the resulting films possess considerable regions, which allow easy access of the guest molecules and analytes to the pores and interlayers.

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[0061] Although preferred embodiments of the invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed but is capable of numerous rearrangements, modifications, and substitutions of parts and elements without departing from the spirit of the invention. 

What is claimed:
 1. A method of electrochemically generating mesoporous metal and metal oxide films from dilute surfactant solutions utilizing self assembly of surfactant-inorganic aggregates at solid-liquid interfaces and werein the working electrode serves as a solid-liquid interface in a plating solution containing surfactant. 